The present invention relates to a method for detecting an object inside a monitored zone, in which at least two sensors respectively monitor at least a part of the monitored zone. The invention is furthermore directed to an apparatus for use in a method of this type.
Methods of this type are in particular used in safety engineering. A check is, for example, made continuously whether an object, in particular a person, sensed inside the monitored zone, intrudes into the danger zone of a machine, for example a robot, working inside the monitored zone. If an intrusion of this type is detected, the machine can be switched off or another suitable safety function can be triggered.
When laser scanners designed as area sensors are used, a separate protected field is usually defined for each sensor with which the zone between the sensor and the machine is monitored for the intrusion of an object. The measured data of each sensor are each supplied separately to an evaluation unit which, for example, switches off the machine when a breach of the protected zone is recognized on the basis of the measured data supplied by a sensor by a comparison with pre-determined desired data. Since the measured data of the sensors are each determined in the respective sensor's own reference system as a rule and are transmitted to the evaluation unit, they must be transformed into the reference system of the evaluation unit prior to the carrying out of the comparison by the evaluation unit in accordance with the arrangement of the sensor and the data format used.
It is problematic in this process that a breach of the protected zone reported by a sensor does not necessarily mean that the sensed object is also breaching the danger zone of the machine. Since the sensor cannot see into the zone disposed between the object and the machine, it is not possible to differentiate whether the part of the object remote from the sensor reaches up to and into the danger zone of the machine or not. For safety reasons, the machine must therefore be switched off, when an object is sensed, even if there is actually no breach of the danger zone of the machine. The sensing of the object is thus incomplete with respect to its actual shape, since the “shadow” of the object lying between the object and the machine from the view of the sensor also has to be taken into account in the object sensing.
It is furthermore a disadvantage that a definition always has to be made up to the sensor due to the described cast shadow of the protected zone of the sensors so that a free definition of the protected zone inside the monitored space is not possible.
Finally, the computing time effort in the evaluation unit for the required coordinate transformation is relatively high so that high computing power is in particular required for a time-critical application when a plurality of sensors are used.